Single wall domain arrangement

ABSTRACT

SINGLE WALL DOMAIN MATERIAL FOR WHICH PERMISSIBLE BIAS FIELD RANGES EXIST FOR ONLY NARROW RANGES OF TEMPERATURES HAVE BEEN FOUND TO BE PARTICULARLY USEFUL OVER RELATIVELY LARGE RANGES OF TEMPERATURE WHEN USED WITH A BIASING MAGNET WHICH PROVIDES A BIASING FIELD WHICH VARIES PROPERLY AS A FUNCTION OF

United States Patent Geusic et al.

Jan. 16, 1973 MAGNETIC SINGLE WALL DOMAIN ARRANGEMENT lnventors: Joseph Edward Geusic, Berkeley Heights; Le Grand Gerard Van Uitert, Morris Twp.. Morris County, both of NJ.

Bell Telephone Laboratories Incorporated, Murray Hill, Berkeley Heights, NJ.

Assignee:

Filed: Dec. 22, 1971 vs. c1 ..340 174 TF, 340/174 MA,

340 174 AG 1m. (:1 ..(;11 11/14, 61 lc 5/04 Field of Search ...340/l74 TF, 174 MA, 174 AG [56] References Cited OTHER PUBLlCATlONS lBM Technical Disclosure Bulletin, Thermal Manipulation of Bubble Domains" by Gambino et al.. Vol. l3, No.7, l2/70, p. l788l790.

Primary ExaminerStanley M. Urynowicz, Jr. Attorney-R. J. Guenther et al.

[57] ABSTRACT 9 Claims, 7 Drawing Figures 5| f i 57% 1; n 1.14 ,56

' 1] m1 llr IL s4 e 58 53 PATENTEUJAN 16 um 3.711.841

SHEET 1 U? 3 H FM. 2

INPUTY 4 PULSE SOURCE 2 UTIIRZlI'lITQN IN PLANE FROM IS TO 22 FIELD SOURCE BIAS 60] FIELD SOURCE I CONTROL CIRCUIT lld. He Hr Ullllllu PAIENTEDJM 161975 3.111.841

SHEET 3 UF 3 FIG. 4

1 FIG. 7 5

so COLLAPSE 1 f 6! I 62 STRIP OUT l k g 64 I IN PLANE FIELD STRENGTH MAGNETIC SINGLE WALL DOMAIN ARRANGEMENT FIELD OF THE INVENTION This invention relates to magnetic storage arrangements and, more particularly, to such arrangements which store information as patterns of single wall magnetic domains.

BACKGROUND OF THE INVENTION A single wall domain is a magnetic domain characterized by a single domain wall which closes upon itself in the plane of a layer of magnetic material in which it can be moved. Inasmuch as the wall closes on itself, the domain is self-defined and is free to move anywhere in the plane. Domains of this type are disclosed in US. Pat. No. 3,460,1l6 of A. H. Bobeck-U. F. Gianola-R. C. Sherwood-W. Shockley issued Aug. 5, 1969.

A layer of magnetic material in which single wall domains can be moved typically comprises an epitaxially grown single crystal film having a preferred direction of magnetization normal to the plane of the film. A domain in such a material is visualized as a right circular cylinder magnetically positive at the top surface of the layer and negative at the bottom forming a magnetic dipole along an axis normal to the plane of movement. When exposed to polarized light, a single wall domain appears as a circular disk relatively dark or light, in contrast to the remainder of the layer, when viewed through an analyzer.

A single wall domain is stable in most suitable films over a (stability) range of diameters which varies from a maximum at which a domain strips out" to a minimum at which the domain collapses, a range in which the maximum and minimum values differ by a factor of about three. A magnetic field of a polarity to reduce the diameter of a domain typically determines an operating diameter in the middle of a bias range which corresponds to the stability range of diameters to ensure the widest possible operating margins in a practical single wall domain arrangement.

Hitherto, it has been prescribed by those skilled in the art that the bias field be maintained at a preselected constant value and that a layer of material suitable for the movement of single wall domains be characterized 'by properties which ensure a stability range which is ideally constant as a function of temperature over a practical temperature range. Inasmuch as the stability range of a layer of selected material varied the operating margins were reduced. The properties of magnetic materials and the relationship of those properties to the stability range are discussed in the Bell System Technical Journal, Vol. 50, No. 3, Mar. 1971, at page 725 et seq., in an article by A. A. Thiele, entitled Device Implications of the Theory of Cylindrical Magnetic Domains.

lively, either leading to improper domain movement and, thus, to decreased margins.

The reduction in margins is noticeable, for example, in operations of the type known as a field-access arrangement. A field-access, single wall domain arrangement includes a periodic pattern of magnetically soft elements, typically of permalloy, which is coupled to the layer in which domains are to be moved. A magnetic field reorienting in the plane of the layer generates in the elements pole patterns which change as the orientation of the field changes. The changing pole patterns produce continuously offset field gradients for moving the domains. The most familiar form of a field-access arrangement employs a repetitive T and bar-shaped pattern of elements, with a period of about three domain diameters, which responds to a magnetic field rotating in the plane of domain movement. US. Pat. No. 3,534,347 of A. H. Bobeck, issued Oct. 13, I970, discloses single wall domain arrangements of this type. Sufficiently large changes in the diameter of domains have been observed'to result in improper movement of domains. Consequently, an important criteria for a suitable domain material has been a relatively constant stability range of domain diameters (or bias field) over a practical temperature range from which a relatively constant operating diameter could be maintained for materials with constant material lengths.

BRIEF DESCRIPTION OF THE INVENTION The present invention is based on the recognition that contrary to prior art teaching to maintain a constant bias field, a bias magnet which supplies a bias field which varies as a simple function of temperature would enlarge the class of materials which are potentially useful for single wall domain arrangements so long as those materials exhibit a stability range which also varies as a like simple function of temperature. In other words, materials with properties which vary as a function of temperature in a manner to provide a stability range which varies in a manner and corresponds to variations in a bias field provided by an associated magnet as temperature varies appear particularly useful in this context.

Indeed, materials have been found with such properties. For example, Y Eu AlFqO and Ca Bi V Fe., have been found particularly well suited for this purpose. Materials of this type are characterized by a magnetization which varies as a simple function of temperature which closely approximates the variation of a permanent magnet of, for example, BaFe O Moreover, these materials as well as a number of related garnets are also characterized by an anisotropy which varies approximately as the fourth power of the moment and thereby produces an essentially invarient material length" or domain diameter over the temperature range of interest. Consequently, a magnetic domain arrangement is realized where a layer in which single wall domains can be moved and a biasing arrangement for determining an operating diameter for domains in the layer are chosen so that each varies as similar functions of temperature within acceptable limits over a relatively large range of temperatures. For a layer of Y Yb Eu AlFe O and a permanent magnet of BaFe O the arrangement operates over a temperature range of 200-400 Kelvin.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a line diagram of a magnetic single wall domain arrangement in accordance with this invention;

FIGS. 2, 3, and 4 are graphs showing the properties of the permanent magnet and domain layer materials as functions of temperature in accordance with this invention;

FIG. 5 is a schematic representation of a permanent magnet structure exhibiting a varying magnetic field as a function of temperature in accordance with this invention;

FIG. 6 is a cross-sectional view of the structure of HO. 5 showing a layer of material in which single wall dbmains can be moved; and

FIG. 7 is a graph of bias field versus in-plane field for various frequencies ofin-plane field.

DETAILED DESCRIPTION FIG. 1 shows a single wall domain arrangement 10 comprising a layer 11 of material in which single wall domains can be moved. Channels along which domains move in layer 11 are defined by a representative pattern of T and bar-shaped elements 12 in a familiar manner.

A representative input for domains is shown at 13 in FIG. I to comprise a magnetically soft rectangular element I4 and an associated conductor 15. Conductor 15 is connected to an input pulse source represented by block 16 in the figure. Copending application, Ser. No. 196,90l of PC. Michaelis filed Nov. 9, I971, discloses an input ofthis type.

A representative detector arrangement is shown at 20. The detector arrangement comprises a magnetoresistance element 21 operative to enlarge a domain moves along a channel into the position of element 21. The element is connected to a utilization circuit 22 to which it applies a signal representative of the presence of a domain. In operation, a current is applied to element 21 (pulsed or d.c.) to effect an output signal. Arrangements of this type are disclosed in copending application, Ser. No. 140,894 of A. H. Bobeck, filed May 6, 1971.

Domains selectively introduced at input I3 are moved along the propagation channel to detector in response to a magnetic field rotating in the plane of layer 11. A source of such a field is represented by block 25 of FIG. I.

In practice, a domain in the arrangement of FIG. 1 is maintained at a nominal operating size by a bias field supplied by a source represented by block 26 of FIG. 1. Any one of a variety of suitable sources is suitable for biasing purposes. For example, a permanent magnet, or an exchange coupled film are suitable to this end. Alternatively, a current driven coil may be adapted for providing the bias field.

In any case, in accordance with prior art teaching, a material for layer 11 is selected to provide domains which are stable over a known (stability) range of bias field values and a particular bias field value is selected to maintain a domain at a nominal operating diameter in that range. The propagation arrangement is designed to have a period about three times the domain operating diameter. Any variation in, for example, domain size leads to operating margin reduction as mentioned hereinbefore.

Criteria for material selection for layer II and for a biasing field source in accordance with prior art teachings are discussed in connection with FIGS. 2 and 3. FIG. 2 shows a graph of bias field H as a function of temperature T over an arbitrary temperature range from 0 to Centigrade, a range which is acceptable for device applications where, for example, air conditioning is not contemplated. The top curve 30, as viewed in the figure, represents the bias field values at which domains collapse at various temperatures. The bottom curve 31 represents the domain strip out values. Both curves can be seen to slope downward for increasingly higher temperatures as is characteristic of suitable materials for layer 11.

In accordance with prior art thinking, practical operation of a single wall domain arrangement is achieved only, for a selected range of temperatures, between the lowest point of curve 30 and the highest point of curve 31. Horizontal lines 32 and 33 in FIG. 2 delineate the operational area for the range of temperatures contemplated. An acceptable change in bias field AH is specified in this manner, the maximum temperature range being defined for AH 0. An acceptable bias field range, of course, may be appreciated to be restricted to a small percentage of the range of bias field at a given temperature as a practical matter. For typical operating bias field values of about 100 oersteds, i 20 oersteds define the collapse and strip out points, respectively. The change AI-I in this instance typically may be less than about a few oersteds and possibly less than a small fraction of an oersted depending on familiar design trade-off considerations.

FIG. 3 shows a graph of magnetic field H versus temperature for a permanent bias magnet for a temperature range of 0 to 100 C. The curve 34 is shown in a worst case configuration (to emphasize a point) to rise for increasingly higher temperatures. The change in bias field AH is defined by the highest and lowest points on the curve intersected by horizontal lines 35 and 36, respectively, in the figure. The change in bias field AH may be appreciated to be restricted to a range consistent with the permissible change AH of FIG. 2 typically a few oersteds or less variation with temperature, the usable margins being AH,,-AH

The prescription for suitable materials for layer 11 and for a suitable biasing magnet in this prior art context is clear. A magnetic material must be selected with properties (such as magnetization M, wall energy, anisotropy, etc.) so that the range of bias field values between collapse and strip out varies only negligibly with temperature T over an acceptable range of temperature values. The prescription for suitable biasing magnets is that the field produced by the magnet be essentially constant over the accepted temperature range.

Within these constraints, the search for suitable materials both for the magnet and for layer II is limited. Typical suitable materials for layer 11 under these constraints are shown in Table I.

Suitable bias magnet materials are shown in Table II.

TABLE II Material AH =(l/M) (dM/dT) Quench hardened steel Vicalloy (38 Fe, 52 Co, 100V) Alnico 5 (8A1, I4Ni, 24Co, 3Cu,

SlFe) Such materials typically can be operated over a range of about 50 C.

The foregoingprior art prescriptions were for ideal materials only approximated in practice. Of course, any selected magnet material exhibits some change in H M as a function of temperature. By the same token, any selected material for layer 11 also exhibits some change as a function of temperature. On occasion, the changes have not been inconsistent with one another resulting in a better than expected operational range. Nevertheless, the prescription for the materials suitable for layer II imposed a severe constraint on those charged with the responsibility to produce suitable domain materials. The most attractive materials used with an ideally constant bias field have been useful over a range of temperatures of about 70C from 0 to 75C.

In accordance with the present invention the constraints on the classes of materials suitable for layer II or for the biasing magnet are considerably relieved. Specifically, the materials for the biasing magnet and for layer 11 are chosen such that H M and H B respectively vary as like functions of temperature. In accordance with this prescription, materials for layer 11 may be selected from a class where none of the members of the class is characterized by a positive AH as shown in FIG. 2. In other words, even for an assumed ideal bias field source which provides a constant bias field independent of temperature, none of the members of the class would be useful over any significant temperature range.

A typical material from this class is represented in FIG. 4 by curves representing relatively sharply decreasing values of H with increasing temperature leading to a negative (viz: nonexisting) AH as indicated by the horizontal lines 32a and 33a corresponding to lines 32 and 33 of FIG. 2. For such materials a positive range AH AI-I exists over only a negligibly small temperature range as is clear from the figure. Typical materials which lie within the present class, unsuitable for single wall domain arrangements in accordance with the prior art prescription, comprise rare earth iron oxides which have no compensation points and have Curie temperatures above the highest contemplated operating temperature often as high as about 500 or 600 Centigrade, and materials which have stress (or strain) induced uniaxial anisotropy out of the plane of the layer. A class of suitable materials with growth induced anisotropy is disclosed in copending application, Ser. No. 204,226 filed Dec. 2, l97l for W. A. Bonner, J. E. Geusic, and L. G. Van Uitert. Table III shows illustrative materials suitable in accordance with this invention, where l/M (dM/dT) is the percent change of magnetization with temperature.

TABLE III Material Substrate fl 1 Growth Induced La L1 4 rz n s m 0.2 %/C Lu Eu Ga Fe O Gd Ga O 0.2 %/C Lu Eu Al F6 0 YGa O 0.2 %/C Stress Induced mi tz om 4.l l2 a 1.1 1.11 I Gd,, ,,Y Yb,AlFe O,, Gd3Ga5O 0. l 5%/C Strain induced anisotropy arises from a choice of materials with lattice constants different from that of the substrate on which they are grown. Epitaxial growth of the film occurs at elevated temperature leading to a layer 11 under tension or compression when the temperature is later reduced to room temperature in accordance with well-understood considerations.

A biasing magnet cooperating with a layer 11 of a material from the class prescribed in accordancewith this invention is designed to exhibit a bias field which varies with temperature as does the material of layer 11, a function of temperature ideally as represented by line 40 of FIG. 4. Line 40 is intermediate lines 30 and 31 of FIG. 2. In practice, the ideal curve is approximated by a characteristic represented by line 45 or 46 in FIG. 4 leading to a AH corresponding to that shown in FIG. 3. The allowable change in bias field is about i 5 percent.

A suitable bias field source is represented by a block designated 26 in FIG. 1 as has been stated hereinbefore. A type of magnet known as a Watson magnet particularly well suited (as source 26) for providing a uniform bias field is shown in detail in FIGS. 5 and 6. The magnet comprises high permeability (typically of permalloy) end plates 51 and 52 as shown in FIG. 5 with plugs 53 and 54 of a selected material to provide an H which tracks the material selected for layer II in any specific arrangement. The separation between the plates is controlled by a screw adjustment as indicated in the figure.

FIG. 6 represents a cross section of the magnet of FIG. 5. The arrows in the figure represent the flux lines and can be seen to represent a uniform field within the area bounded by plugs 53 and 54. The intensity of the field is adjusted (via the screw adjustment) by changing the air gaps 56 and 57 in FIG. 6. Layers Ila through lli, formed on suitable substrates, typically are mounted on a ceramic fixture 58 for insertion into the area between plugs 53 and 54 as shown in FIG. 6.

The shape of curve 40 in FIG. 4 produced by such a magnet is determined by the materials used in the plugs 53 and 54. The position of the curve is determined by the adjustment of air gaps S3 and 54 as is consistent with prior art thinking. Materials which are particularly well suited in this respect are shown in Table IV.

In a first example in accordance with this invention, an epitaxially grown garnet film of Ca Bi V Fe, ,O exhibiting stress induced anisotropy is selected for layer 11 and ceramic BaFe O, is selected as the material for plugs 53 and 54. The plugs (as well as layer 11) exhibit a decrease of 0.2 percent per degree in field strength as temperature varies from 0 lO0 C. with the gaps adjusted to give a strength of I00 oersteds at 50 C. Layer 11 exhibits essentially the same percentage decrease in desirable bias as shown in FIG. 4. In accordance with the prior art, a layer 11 comprising a film of Ca, Bi, V ,,Fe., ,O, would have only a small useful range of operation. The arrangement exhibits about a :15 percent change in domain diameter and operates over a temperature range greater than from 50 to +150C.

In a second example an epitaxially grown film of Y, Eu exhibiting growth induced anisotropy is selected for layer 11 and ceramic BaAlFe O is employed as the material for plugs 53 and 54 to provide an average 0.2%/C drop in field. Domain diameter in such arrangements varies about il0% over a temperature range of from 0 to 100C.

FIG. 7 shows a family of curves representing bias field H as a function of the rotating field strength at different rotating field frequencies, a dc. value represented by curve 60, a relatively low frequency represented by curve 61 and a relatively high frequency represented by curve 62. The area defined by each curve represents the associated operating margins. Curve 62 corresponds to the frequency at which domain movement becomes limited by the mobility of the material of layer 11. Vertical broken lines 63 and 64 represent rotating field values below which the coercivity of the overlay (viz: T-bar shaped) pattern of permalloy has sufficient coercivity to resist switching and above which the rotating field values saturate the pattern, respectively.

If it is recalled that increasingly higher values of bias field H correspond to increasingly smaller diameter domains, it becomes clear from FIG. 7 that margins are reduced for increasingly smaller domains as rotating field frequency increases. The reason for this is that for a fixed field gradient, the force on a domain is a function of domain radius as is well known. Consequently, the smaller the radius (diameter) the slower the domain moves.

In this context, an optimum bias field source varies as a function of temperature in a manner to maintain domain size constant. This implies that the material length" l (I (o' )/4'rrM,,)2, where 0' is the wall energy) be relatively constant versus temperatures over as wide a temperature range as can be achieved. For optimum film thicknesses (with the materials Y, -,Eu, -,A lFeO and Ca, Bi, -,,V ,,Fe,, ,O listed in Table III) equal to about four times the characteristic material length, a permissible AH (see FIG. 4) for curve 40 is :5 percent which maintains domain diameters to within an acceptable :25 percent of the nominal values.

Sources 16 and 25 and circuit 22 are connected to a control circuit 60 shown in FIG. I for synchronization and actuation. The various sources and circuits may be any such elements capable of operating in accordance with this invention.

What has been described is considered merely illustrative of the principles of this invention. Therefore, various modifications thereof can be derived by those skilled in the art in accordance with those principles within the spirit and scope of this invention. For example, exchange coupled films are known to provide bias fields for constricting domains in a contiguous film. It is contemplated, in accordance with this invention, that a domain layer and an exchange coupled film be chosen of materials to exhibit like variations in their pertinent characteristics as represented in FIG. 4.

What is claimed is:

l. A magnetic arrangement comprising a layer of material in which single wall domains can be moved, said domains being stable in said material only at operating temperatures in a first range in the presence of a bias field varying only negligibly from a first value, said layer being characterized by a variation in the range of bias field values for which domains exist therein as a first function of temperature over a second range greater than and including said first range such that no domains are stable in said layer over said second range outside of said first range in the presence of a bias field of about said first value, and means for supplying said bias field comprising a first material which provides a bias field which is a function of temperature over said second range such that at each temperature therein the bias field is of a value at which domains are stable in said layer.

2. A magnetic arrangement comprising a layer of material in which single wall domains can be moved, said domains being stable in said layer only at operating temperatures in a first range in the presence of a bias field varying only negligibly from about a first value, said layer being characterized by a variation in the range of bias field values for which domains exist therein as a first function of temperature over a second range greater than and including said first range such that no domains exist in said layer over said second range of temperatures outside of said first range in the presence of a bias field of about said first value, and means for supplying said bias field comprising a first material which provides a bias field which varies essentially as said first function of temperature over said second range of temperature.

3. A magnetic arrangement in accordance with claim 2 wherein said layer of material comprises a layer of garnet characterized by the absence of a compensation point and a Curie temperature higher than temperatures in said second range.

4. A magnetic arrangement in accordance with claim 2 wherein said means for providing said bias field includes layers of magnetically soft materials separated by plugs of said first material which define therebetween an area for said layer of material in which said domains can be moved whereby a uniform bias field is provided for the last-mentioned layer.

5. A magnetic arrangement in accordance with claim 2 wherein said layer of material comprises an epitaxially grown film on a nonmagnetic substrate.

6. A magnetic arrangement in accordance with claim 3 wherein said layer of material comprises an epitaxially grown film on a nonmagnetic substrate.

7. A magnetic arrangement in accordance with claim 5 wherein said layer and said substrate have crystal properties sufficiently different that said layer exhibits a 9. A magnetic arrangement in accordance with claim sktlrain induced uniaxial anisotropy out of the plane of 8 wherein said layer comprises Cam Bil-2 VM Fe 0'2 t e ayer.

8. A magnetic arrangement in accordance with claim 2 wherein 'said layer comprises nominally CaBiVFe O 5 and said means for supplying said bias field comprises a ceramic BaFe O UNITED STATES PATENT AND TRADEMARK OFFICE @ETIFICATE @F CORRECTION 3,711,8H1 DATED January 16, 1973 Q PATENT NO.

INVENTOR(S) Joseph Edward Geusic and LeGrand Gerrard Van Uitert it is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6, line 8, "Lu Eu Al Fe O and "YGa O 5 12 should read Lu Eu AlFe O and --Y Ga O b H H llne l0, Gd So Ga should read Gd e o H H llne ll, Gd Y Yb AlFe O should read Gd Y Yb AlFe O p Signed and Scaled this ourth 21 mm f D of May 1976 Arrest:

RUTH C. MASON C. MARSHALL DANN 011m (mnmissiom-r ()fIQL I1IS and p 

2. A magnetic arrangement comprising a layer of material in which single wall domains can be moved, said domains being stable in said layer only at operating temperatures in a first range in the presence of a bias field varying only negligibly from about a first value, said layer being characterized by a variation in the range of bias field values for which domains exist therein as a first function of temperature over a second range greater than and including said first range such that no domains exist in said layer over said second range of temperatures outside of said first range in the presence of a bias field of about said first value, and means for supplying said bias field comprising a first material which provides a bias field which varies essentially as said first function of temperature over said second range of temperature.
 3. A magnetic arrangement in accordance with claim 2 wherein said layer of material comprises a layer of garnet characterized by the absence of a compensation point and a Curie temperature higher than temperatures in said second range.
 4. A magnetic arrangement in accordance with claim 2 wherein said means for providing said bias field includes layers of magnetically soft materials separated by plugs of said first material which define therebetween an area for said layer of material in which said domains can be moved whereby a uniform bias field is provided for the last-mentioned layer.
 5. A magnetic arrangement in accordance with claim 2 wherein said layer of material compriSes an epitaxially grown film on a nonmagnetic substrate.
 6. A magnetic arrangement in accordance with claim 3 wherein said layer of material comprises an epitaxially grown film on a nonmagnetic substrate.
 7. A magnetic arrangement in accordance with claim 5 wherein said layer and said substrate have crystal properties sufficiently different that said layer exhibits a strain induced uniaxial anisotropy out of the plane of the layer.
 8. A magnetic arrangement in accordance with claim 2 wherein said layer comprises nominally CaBiVFe4O12.
 9. A magnetic arrangement in accordance with claim 8 wherein said layer comprises Ca1.8 Bi1.2 V0.9 Fe4.1 O12 and said means for supplying said bias field comprises a ceramic BaFe12O19. 